Piling is the deposit of unwanted, excess ink and/or paper residue on either the plate or blanket. It is a natural result of the offset printing process, which places ink, paper, plate and blanket in close physical contact under high pressure.
Image area piling, appearing on the plate, interferes with the ink transfer process, causing a gradual deterioration in print quality. Non-image area piling generally accumulates on the blanket, and if permitted to build-up will also reduce quality. This piling has to be removed periodically with solvent or an emulsion of water and solvent, either by hand or an automatic blanket washing system. Without resolution of piling problems, the printer is left with unplanned waste, in the form of printed material, which has to be discarded.
Paper piling is the most common type of non-image area piling seen in the pressroom. It usually comes from Tinting loose paper and particles from either the edges or surface of paper, which accumulates on the blanket. As paper piling builds up, print quality deteriorates, usually seen as lose of highlight dots or gradual lightening in the solid areas.
There are several contributing factors that influence the rate of piling:                1. The amount of water being carried on the plate/blanket. Running too dry generally increase the rate of piling        2. The speed of the inks. Inks formulated with faster oils may tend to dry out and pile rapidly        3. The type of plate used—smooth grain plates usually pile less        4. Paper surface—loose fiber may be pulled off and added to the accumulated ink resin        5. The lubricating ability of the fountain solution        